Self-study
Conclusion Learning how to build a nest plays an important part in the breeding success of birds. ███ ████████ ███ ████ ███ ████████ ███ ███████ ██ █ ██████ ██ ██████████ ██ ███████ ██████████ ██████ ██ █████ ████ █████ ███████ ███ ███ █████ ████ ███ ████ ██████████ ██ ████████ ████ ███ █████ ██████ ███ ████ ████ ██████████ ████ ████ ██████████ ███ █ ████ ██████ ████ ██████ ██ █ ████ ██████ ██ ████ ███ █████████ █████ ███████████ ████ ███ █████ ████████ ██ ██████ ███ █████ █████ ████ ████ █████ ███ █████ ██ ██ █████████ ██ █████ ███ ██████████ ████ ████ ███████ ██ █████ ███████ ███████████
Structure: Phenomenon-Hypothesis
The stimulus leads with the hypothesis that learning to build a nest is an important factor in birds' breeding success. This is supported by an example of a phenomenon observed by Dr. Snow: blackbirds nesting for the first time are less successful in breeding than older blackbirds are, and also less successful than they themselves will be a year later. The author rules out the possibility that this phenomenon is caused by changing size and strength, since blackbirds are fully grown once they leave the nest. The author concludes that the birds must "benefit" from their nesting experience — a conclusion that supports the hypothesis offered in the first sentence.
Analysis: Ways to Weaken
From a correlation between nest-building experience and breeding success, the author infers causation: nest-building experience leads to greater breeding success. As with most causal arguments, a great way to weaken this argument would be to provide an alternative explanation for the variations in breeding success. While the author rules out size and strength as alternative explanations, there could still be other factors at play — for instance, maybe the key is not that birds need time to learn how to build nests, but that they need time to learn how to perform mating displays. So an answer choice that suggests an alternative causal explanation would be a good choice for this Weakening question.
18.
Which one of the following, ██ █████ █████ ████ ██████ ███ █████████
a
Blackbirds build better █████ ████ █████ ██████
Incorrect. This answer choice is trying to suggest that the example of blackbirds doesn't actually support the conclusion about birds in general. This would be an effective way to weaken the argument if it actually succeeded — but to do that, this answer choice would need to provide more information about an actually relevant difference between blackbirds and other birds. Just because blackbirds build "better" nests than other birds doesn't imply on its own that blackbirds can't be representative of other nest-building birds.
b
The capacity of ██████████ ██ ███ ██████ ████ █████████ ████ ████ ██████████ █████ ██████ ███ █████ ███ █████ ██ █████████████
Correct. Instead of nest-building experience being the main factor in why blackbirds are less successful in breeding early on, this answer choice suggests that the key factor is biological: blackbirds' capacity to lay viable eggs increases in successive years of reproduction. By providing an alternative explanation for the variations in breeding success among blackbirds, (B) effectively weakens the argument.
c
The breeding success ██ █████ ███████ ███ ███ ██████ ████ ██ ███████ ████ ████ ██ █████ ███████ ███ ███ █████ █████
Incorrect. This is exactly what we would expect given the author's hypothesis. If nest-building experience is a key factor in birds' breeding success, then it makes sense that they would have more success breeding when they are nesting for the second time — i.e., when they have some nest-building experience — than when they are nesting for the first time, when they have no nest-building experience. This strengthens, not weakens, the argument.
d
Smaller and weaker ██████████ █████ ████ ██ ████████████ ██ ██████ ███ ████████ ███████████
Incorrect. The author has already ruled out the possibility that size and strength are alternative explanations for variations in blackbirds' breeding success. This answer choice just confirms something the author has already said, so it certainly doesn't weaken the argument.
e
Up to 25 ███████ ██ ███ █████ ███ ██████ ██ █████████ ██████ ████ █████ ██ █████
Incorrect. The author's argument is about birds that have already begun nesting, not about birds before they start nesting. Knowing that some birds are killed before they start nesting doesn't undermine the author's hypothesis that nesting experience causes greater breeding success.